Occasions arise where laterals are needed to extend from a main bore or another lateral to enhance production from a given formation. The process for doing this is usually to anchor and orient a whipstock that has a milling assembly associated with it when run in. The whipstock is a long body with a ramp that has to be oriented in the desired direction of the casing exit to be produced. Once the ramp is oriented as needed and the anchor is set the milling assembly is pushed down or pulled up to break away from a lug above the whipstock ramp. The milling assembly is then rotated to progress along the whipstock ramp surface and cut through the wall of the surrounding tubular to make a window. In multilateral applications, a tubular lateral is extended through the window with suitable seals to isolate the main bore and a lateral bore through the window made by the mills.
The milling assembly is a collection of two or more mills with a pilot mill leading followed by one or more elliptically shaped mills that lengthen the window made by the pilot mill. In the past the watermelon mill was a unitary construction of an elliptical shape on a tubular mandrel having upsets and threads at opposed ends to be made up into a tubular string for rotation to mill the window. These mills were subject to stress concentrations in the welded locations of the mill at the transition to the flexible shaft that extends from opposed ends. In larger sizes of watermelon mills the dimensional difference between the mandrel and the cutting structure diameter is far greater than in the smaller sizes making the welded transition between the mandrel and the cutting structure assembly a location for stress failure. In an effort to eliminate welding in the transition from the mandrel to the cutting structure and to make redressing the mills for reuse simpler, the present invention provides a cutting structure sleeve held in position in a variety of ways using threaded connections so that the assembly becomes more flexible to minimize or eliminate stress failures in the larger sizes and to make redressing the mill a simple matter of threaded connection disassembly. The components can be assembled in a variety of ways to accomplish these purposes and some options for such assemblies are described in the detailed description of the preferred embodiment and the associated drawings. Those skilled in the art will recognize, however, that the full scope of the invention is to be found in the appended claims.
Generally relevant to cutting structures detachable from mandrels and dropped in the hole are US 2007/0256867 and US 2013/0328275. U.S. Pat. No. 9,097,073 shows articulated blades that can be disassembled from a mill body.